Deane’s question: “I wonder if it’s acceptable to write spacecrafts (or aircrafts) instead of using the singular. I thought ‘craft’ was similar to the use of sheep – one word functions for both singular and plural.”

BizWritingTip response: Again, another example of our words changing. Most dictionaries, e.g., Oxford, Merriam-Webster, and Cambridge Dictionaries Online, agree with you. “Craft” refers to both singular or plural nouns – the same as the word “sheep.”

Example (most dictionaries)
The pilot turned the spacecraft toward earth.
The pilot has spent time on several spacecraft.

However, in researching this question, I found that Webster’s College Dictionary and Wiktionary accept both “spacecraft” and “spacecrafts” as the plural noun.

Example
Seven spacecrafts were successful in landing on Mars.

Therefore, in some situations you might get away with adding an “s” to spacecraft. However, if you stick with “spacecraft” regardless of whether it is singular or plural, you will always be correct.